Boring bar with means to reduce vibrations



March 10, 1970 BORING BAR WITH MEANS TO REDUCE VIBRATIONS R. s. HAHN 3,499,350

Filed April 1, 1968 25 SUPPORT FIG. 2

INVENTOR. ROBERT S. HAHN ATTORNEY United States Patent US. Cl. 7758 6 Claims ABSTRACT OF THE DISCLOSURE This invention has to do with the boring bar and, more particularly, the machine tool apparatus in which unwanted vibrations are reduced to a minimum, wherein the boring bar shaft extends from both sides of a mounting flange having the cutting tool mounted on one end of the shaft and a vibration damping means located at the other end of the shaft.

BACKGROUND OF THE INVENTION In the operation of machine tools for the generation of a surface of revolution, it is common practice to mount a single point tool on the outer end of a boring bar. This bar is mounted on the end of a spindle which is rotated at high speed. Unfortunately, the fact that the cutting force is necessarily applied at the end of a rotating cantilever means that unwanted vibration appears ,under certain circumstances. Such vibrations can cause poor finish on the workpiece surface and difficulties in maintaining cutting rates and geometry. These and other difiiculties experienced with the prior art devices have been obviated in a novel manner by the present invention.

It is, therefore, an outstanding object of the invention to provide a boring bar in which unwanted vibrations are reduced to a minimum.

Another object of this invention is the provision of a boring bar with which the cutting action takes place without chatter.

A further object of the present invention is the provision of a boring bar capable of generating a surface of revolution with excellent finish.

With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.

SUMMARY OF THE INVENTION In general, the present invention has to do with a boring bar having an elongated shaft adapted to carry a cutting tool at one end, having a diaphragm extending transversely of the shaft in the center portion thereof, the diaphragm being fixed on its periphery and connected in its central portion to the shaft, and having means located at the other end of the shaft for damping vibrations.

More specifically, the said other end of the shaft is provided with a cylindrical surface, and the damping means consists of a block having a bore extending coaxially of and slightly spaced from the cylindrical surface and of a viscous fluid which lies between the bore and the cylindrical surface. The block is adjustable to various positions along the shaft. The diaphragm is made very stiff relative to the tool end of the shaft and the masses and spring constants of the two halves of the shaftare selected so that the natural frequencies are the same.

BRIEF DESCRIPTION OF THE DRAWINGS The character of the invention, however, may be best understood by reference to one of its structural forms, as illustrated by the accompanying drawings, in which:

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FIG. 1 is a sectional view of a boring bar incorporating the principles of the present invention, and

FIG. 2 is a schematic representation of the boring bar demonstrating the theoretical relationships in the system.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, the boring bar, indicated generally by the reference numeral 10, is shown as having an elongated shaft 11 having a cutting tool 12 mounted at one end. From the center of the shaft extends a diaphragm 13 having at its periphery a relatively heavy rim 14 which is bolted to one end of a tubular spindle 15. The spindle is suitably mounted for high speed rotation in bearings 16.

The spindle is provided with an interior bore 17 into which is threaded a block 18 having a coaxial bore 19. This bore is coaxial to and spaced from a cylindrical surface 21 on the end of the shaft opposite the end carrying the tool 12. The diameter of the bore 19 is only slightly larger than that of the surface 21, thus defining between them a thin annular passage 22. The chamber 23 defined by the diaphragm 13, the bore 17, and the block 18 is filled with a viscous fluid which extends through the passage 22 into the remainder of the spindle.

FIG. 2 shows the relationship of the parts. Considering the connecting parts of the shaft as the support 24 (indicated :by the dotted line area in FIG. 1), the diaphragm 13 is indicated as a spring having a spring constant k The mass m of the tool end of the shaft is connected to the support by its spring constant k while the mass m of the other end of the shaft is suspended from the support by its spring constant k The fluid in the passage 22 is represented by the damper 25. When the tool 12 is performing a cutting operation, vibrations may start in the tool end of the shaft; it acts as a cantilever about the support 24. The vibrations feed into the other end of the shaft. Since the frequency of either half of the shaft is defined by the equation:

where i E 1 21r When the diaphragm 13 is selected with a stiffness or spring constant k much larger than k, or k (say, 10 times as large), the vibrations will feed from one mass to the other. The selection and adjustment of the damper 25, therefore, acts to inhibit the vibrations in the mass m and this operates to dampen the vibrations in the tool end of the shaft.

It is obvious that minor changes may be made in the form and construction of the invention without departing from the material spirit thereof. It is not, however, desired to confine the invention to the exact form herein shown and described, but it is desired to include all such as properly come within the scope claimed.

The invention having been thus described, what is claimed as new and desired to secure by Letters Patent is:

1. A boring bar, comprising (a) an elongated shaft adapted to carry a cutting tool at one end,

(b) a diaphragm extending transversely of the shaft in the center portion thereof, the diaphragm being fixed on its periphery and connected in its central portion to the shaft, and

(c) means located at the other end of the shaft for damping vibrations.

2. A boring bar is recited in claim 1, wherein the said other end of the shaft is provided with a cylindrical surface, wherein the said means consists of a block having a bore extending coaxially of and slightly spaced from the cylindrical surface, and wherein a viscous fluid lies between the bore and the cylindrical surface.

3. A boring bar is recited in claim 2, wherein means is provided to adjust the amount by which the said bore is coextensive with the cylindrical surface.

4. A boring bar as recited in claim 1, wherein the product of the mass of the portion of the shaft on one side of the diaphragm and the spring constant of the portion of the shaft on the other side equals the product of the mass on the other side and the spring constant of the portion on the said one side.

5. A boring bar as recited in claim 1, wherein the spring constant of the diaphragm isin the order of ten times thes'pring constant of the portion of the shaft on the side of the diaphragm including the end adapted to carry the ,cutting tool.

6. A boring bar as recited in claim 1, wherein the diaphragm is integral with the shaft.

References Cited UNITED STATES PATENTS 3,174,404 3/1965 Findley 90-24 GERALD A. DOST, Primary Examiner 

